Kill Command in Linux

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Linux is a great and advanced operating system, but it is not perfect. Once in a while, some applications may start behaving erratically and become unresponsive or start consuming a lot of system resources. Unresponsive applications cannot be restarted because the original application process never shuts down completely. The only solution is to either restart the system or kill the application process.

There are several utilities that allow you to terminate errant processes with kill the being the most commonly used.

kill Command

kill is a shell builtin in most Bourne-derived shells such as Bash and Zsh. The command behavior is slightly different between the shells and the standalone /bin/kill executable.

Use the type command to display all locations on your system containing kill:

type -a kill
kill is a shell builtin
kill is /bin/kill

The output above tells that the shell builtin has priority over the standalone executable, and it is used whenever you type kill. If you want to use the binary, type the full path to the file /bin/kill. In this article, we will use the Bash builtin.

The syntax of the kill command takes the following form:

kill [OPTIONS] [PID]..

The kill command sends a signal to specified processes or process groups, causing them to act according to the signal. When the signal is not specified, it defaults to -15 (-TERM).

The most commonly used signals are:

  • 1 (HUP) - Reload a process.
  • 9 (KILL) - Kill a process.
  • 15 (TERM) - Gracefully stop a process.

To get a list of all available signals, invoke the command with the -l option:

kill -l

Signals can be specified in three different ways:

  1. Using number (e.g., -1 or -s 1).
  2. Using the “SIG” prefix (e.g., -SIGHUP or -s SIGHUP).
  3. Without the “SIG” prefix (e.g., -HUP or -s HUP).

The following commands are equivalent to one another:

kill -1 PID_NUMBER
kill -SIGHUP PID_NUMBER
kill -HUP PID_NUMBER

The PIDs provided to the kill command can be one of the following:

  • If PID is greater than zero, the signal is sent to the process with ID equal to the PID.
  • If PID is equal to zero, the signal is sent to all processes in the current process group. In other words, the signal is sent to all processes belonging to the GID of the shell that invoked the kill command. Use ps -efj command to view the process group IDs (GIDs).
  • If PID is equal to -1, the signal is sent to all processes with the same UID as the user invoking the command. If the invoking user is root, the signal is sent to all processes except init and the kill process itself.
  • If PID is less than -1, the signal is sent to all processes in the process group eq with GID equal to the absolute value of the PID.

Regular users can send signals to their own processes, but not those that belong to other users, while the root user can send signals to other user's processes.

Terminating Processes Using the kill Command

To terminate or kill a process with the kill command, first you need to find the process ID number (PID). You can do this using different commands such as top, ps, pidof and pgrep.

Let's say the Firefox browser has become unresponsive, and you need to kill the Firefox process. To find the browser PIDs use the pidof command:

pidof firefox

The command will print the IDs of all Firefox processes:

6263 6199 6142 6076

Once you know the processes numbers, you can terminate all of them by sending the TERM signal:

kill -9 2551 2514 1963 1856 1771

Instead of searching for PIDs and then terminating the processes, you can combine the above commands into one:

kill -9 $(pidof firefox)

Reloading Processes Using the kill Command

Another common use case for kill is to send the HUP signal, which tells the processes to reload its settings.

For example, to reload Nginx, you need to send a signal to the master process. The process ID of the Nginx master process can be found in the nginx.pid file, which is typically is located in the /var/run directory.

Use the cat command to find the master PID:

cat /var/run/nginx.pid
30251

Once you found the master PID reload the Nginx settings by typing:

sudo kill -1 30251

The command above must be run as root or user with sudo privileges.

Conclusion

The kill command is used to send a signal to processes. The most frequently-used signal is SIGKILL or -9, which terminates the given processes.

If you have any questions or feedback, feel free to leave a comment.